{"id":966,"date":"2014-03-29T04:06:45","date_gmt":"2014-03-29T11:06:45","guid":{"rendered":"http:\/\/bestperformancegroup.com\/?page_id=966"},"modified":"2016-02-27T18:32:09","modified_gmt":"2016-02-28T01:32:09","slug":"shoulder-complex-kinematic-considerations","status":"publish","type":"page","link":"http:\/\/bestperformancegroup.com\/?page_id=966","title":{"rendered":"Shoulder Complex – Kinematic Considerations"},"content":{"rendered":"

Much of the following description of shoulder kinematics is taken from\u00a0Zatsiorsky’s\u00a0Kinematics of Human Motion<\/a>\u00a0as he provides an excellent description and review of this very complex topic.<\/p>\n

Describing kinematics of the shoulder mechanism is not a trivial problem.\u00a0 Usually, articular motion of a distal body segment is defined with regard to the adjacent proximal segment, which is considered fixed.\u00a0 For example, the shank movement in the knee joint is analyzed with regard to the femur.\u00a0 In the shoulder complex, however, the scapula and clavicle are not fixed with the torso; rather they displace underneath the skin during movement.\u00a0 Thus, their orientation is hard to measure, their anatomic position is not well defined, and their relative positions are different for different people.\u00a0 Also, the principal joint movements, such as flexion-extension and abduction-adduction, are not defined for these bones.\u00a0 All of these issues make studying the shoulder complex kinematics very difficult.\u00a0 To avoid these obstacles, many investigators disregard the motion of the scapula and limit the study to the movement of the humerus with regards to the trunk or thorax.<\/p>\n

When discussing shoulder kinematics, the shoulder bone will often be defined with regard to the thorax rather than with relative to a proximal bone. \u00a0When reviewing literature values, it is important to know whether the joint kinematics are calculated relative to a more proximal bone forming the joint or the movement is measured relative to another reference frame, anchored either to the trunk or to the axial skeleton.<\/p>\n

Glenohumeral Joint Kinematics<\/em><\/span><\/p>\n

When considering glenohumeral joint kinematics, the motion of the humerus is measured with respect to the glenoid fossa of the scapula. \u00a0The glenohumeral joint has 3 rotational DOF – flexion\/extension, abduction\/adduction, and internal\/external (or medial\/lateral) rotation.<\/p>\n

\"glenohumdof\"<\/a><\/p>\n

The glenohumeral joint surfaces are very close to spherical and the mating joint surfaces are quite congruent and have radii within a 3 mm difference. \u00a0Because the joint surfaces are spherical and in congruence, the joint kinematics are essentially rotational and the surface-on-surface motion is mainly gliding. \u00a0During arm elevation, the center of the humeral head is approximately at the same place and the surface of the humeral head glides downward on the surface of the glenoid fossa. \u00a0The amount of displacement of the ICR can be considered to be negligibly small. \u00a0As such, the glenohumeral articulation is typically modeled as a ball-and-socket joint.<\/p>\n

However, the radii of curvature of the glenoid and the humeral head are not completely identical. \u00a0This is especially true if the cartilage is neglected and only the bony curvatures are compared. \u00a0The ratio of the radii of curvature of the matching humeral head to glenoid surfaces ranges from 0.89\u20131.09.\u00a0 Thus, in some people, and especially those with degenerative cartilage conditions, the articular surfaces are not exactly congruent.<\/p>\n

A rather complex pattern of glenohumeral movement results from the up to 3 mm difference between the radii of curvature of the humeral head and glenoid articular surface.\u00a0 In abduction, the humeral head migrates upward.\u00a0 During the first 30\u00b0\u00a0of arm elevation in the scapular plane the humeral head moves up by about 3 mm due to a combination of rolling and translation.\u00a0 Beyond the initial 30\u00b0, the humeral head displaces approximately 1 +\/- 0.5 mm up or down each 30\u00b0\u00a0of angular rotation.\u00a0 During external rotation, the humeral head displaces backward and during internal rotation forward.\u00a0 So during the arm cocking phase, the humeral head glides posteriorly on the glenoid fossa, but during the delivery phase it will glide anteriorly.\u00a0 These displacements are mainly due to differences in the curvature of the humeral head and glenoid cavity.<\/p>\n

Due to these curvature differences, the glenoidal center of rotation and the humeral head center of rotation will be different. \u00a0Arm elevation is thus a combined motion of these two elementary rotations that are in opposite directions. \u00a0If the glenohumeral joint was a pure ball-and-socket joint, the joint ICR would be coincident with the two elementary rotations. \u00a0But it is not, and the humeral head rotates around the joint ICR. \u00a0However, most all shoulder kinematics studies model the glenohumeral joint as a ball-and-socket joint to negate these differences.<\/p>\n

Because the glenohumeral joint is modeled as a ball-and-socket joint with 3 rotational DOF, the inherent problems of describing 3D rotations such as\u00a0induced twist<\/em><\/a>\u00a0are often found in the literature for shoulder kinematics. \u00a0Specifically, the reported magnitude of the axial rotation of the humerus is dependent upon the angular convention selected.<\/p>\n

There are two different ways to recognize arm abduction,\u00a0frontal plane abduction<\/em>\u00a0and\u00a0scapular plane abduction<\/em>.\u00a0 The scapular plane is defined as the plane of the scapula in the resting position.\u00a0 During arm motion the scapula changes its orientation and no fixed scapular plane exists.\u00a0 In frontal plane abduction, the range of motion depends on the orientation of the humerus.\u00a0 With the humerus internally rotated, the abduction range is limited by impingement of the greater tubercle of the humerus on the acromium process and is typically between 60\u00b0-90\u00b0.\u00a0 With the humerus externally rotated, the range of motion increases to 90\u00b0-120\u00b0.\u00a0 In scapular plane abduction, the humeral rotation does not influence the range of motion.\u00a0 When the arm is abducted in the plane of the scapula, which is angled between 30\u00b0-45\u00b0\u00a0anterior to the frontal plane, there is much less restriction to motion.\u00a0 The range of active abduction in this plane is on average 107\u00b0-112\u00b0.\u00a0 The range of motion in frontal plane abduction increases from 90\u00b0-120\u00b0\u00a0when the abduction is performed passively (caused by external forces).\u00a0 However, this motion is difficult to separate from an accompanying scapular movement.\u00a0 The range of motion for flexion is on average 120\u00b0\u00a0for both active and passive movement.<\/p>\n

When the movement of the humerus relative to the axial skeleton is desired rather than relative to the scapula, the humerus is the end link of the following kinematic chain: trunk (sternum) -> sternoclavicular joint -> clavicle -> acromioclavicular joint -> scapula -> glenohumeral joint -> humerus. \u00a0Relative to the sternum, the glenohumeral joint is located at a sphere with the clavicle as the radius. \u00a0Orientation of the glenohumeral joint is defined by the position of the scapula.<\/p>\n

Scapular (Scapulothoracic and Acromioclavicular) Joint Kinematics<\/em><\/span><\/p>\n

The main function of the scapular motion is to orient the glenoid cavity for the best humeral contact.\u00a0 Functionally, the scapula is part of a closed kinematic chain formed by three links (the scapula, clavicle, and trunk [sternum]) and the three joints (sternoclavicular, acromioclavicular, and scapulothoracic).\u00a0 Any scapular movement occurs simultaneously in the scapulothoracic and claviculoscapular articulations.<\/p>\n

Not including the humerus, the scapula only has one bony connection, with the clavicle through the acromioclavicular joint.\u00a0 Because the lateral clavicle moves in the sternoclavicular joint, the acromioclavicular joint moves substantially relative to the thorax.\u00a0 Hence, the scapula can rotate around three axes in the acromioclavicular joint and translate together with this joint in two directions. \u00a0These movements are, however, coupled, and the scapula has only four, or maybe even three, DOF.\u00a0 The motion of the scapula over the thorax is not planar and because of that an isolated rotation or translation about an individual axis cannot be performed.\u00a0 For example, when the scapula glides over the rib cage in the lateromedial direction it rotates at the same time around a vertical axis (winging), and when it migrates vertically it rotates around a frontal axis (tipping or tilting).\u00a0 Because of this dependence, any classification of the scapula motion is not very strict.<\/p>\n

\"scapularcoupleda\"<\/a><\/p>\n

In the acromioclavicular joint, there are three angular motions: scapular rotation, winging, and tipping.\u00a0 The primary movement of the scapula is its upward rotation (glenoid tilts upward) and downward rotation (glenoid tilts downward) performed around an anteroposterior axis.\u00a0 The rotation occurs in both the acromioclavicular and scapulothoracic joints, and is often referred to as abduction-adduction.\"scapular-rotations\"<\/a><\/p>\n

Because of the complex character of the rotation, the instantaneous center of rotation (ICR) displaces substantially.\u00a0 Between 0\u00b0-80\u00b0\u00a0of arm elevation the ICR is near the root of the spine of the scapula, between 80\u00b0-140\u00b0\u00a0the center migrates towards the acromioclavicular joint, and beyond 140\u00b0\u00a0the center is at the acromioclavicular joint.<\/p>\n

\"scapularcoupledab\"<\/a><\/p>\n

Rotation of the scapula around a vertical axis is called\u00a0winging<\/em>.\u00a0 It is accompanied by the gliding of the scapula over the contour of the ribs in the scapulothoracic articulation.\u00a0 When this movement is performed jointly with anterior movement of the lateral end of the clavicle it is called\u00a0protraction<\/em>\u00a0of the scapula.\u00a0 Protraction results in the translation of the scapula around the curved chest away from the vertebral column.\u00a0 The opposite movement is\u00a0retraction<\/em>.\u00a0 In a retracted position, the scapula approaches the vertebral spines with the vertebral border approximately parallel to the spinal column.\"protraction\"<\/a><\/p>\n

Rotation of the scapula around a frontal axis is called\u00a0tipping<\/em>\u00a0or\u00a0tilting<\/em>.\u00a0 This is essentially an internal rotation of the scapula, a movement of the inferior tip of the scapula away from the thorax.\u00a0 An internal rotation of the scapula, during which the superior tip of the scapula moves posteriorly, takes place when arm elevation is accompanied by external rotation of the humerus.<\/p>\n

\"scaptipping\"<\/a><\/p>\n

When people shrug their shoulders, the scapula is elevated.\u00a0 It is depressed when people drop their shoulders.\u00a0 Because all of the motions of the scapula are coupled, the elevation and depression of the scapula, which are considered predominantly translator movements, are accompanied by scapular rotations.\"scapelevate\"<\/a><\/p>\n

A quantitative description of scapular motion is far from simple.\u00a0 Scapular movement is hard to register and difficult to describe.\u00a0 Any movement is essentially 3D and combines translation with rotation.\u00a0 Rotation of the scapula around an axis, such as during tipping, changes the orientation of the other axes of rotation, i.e., the axis of winging is not vertical anymore.\u00a0 Hence, the classic anatomical angles are not determined flawlessly and Euler\u2019s angles should be used to define scapular orientation.\u00a0 The scapular motion is not independent; it is an element of the shoulder girdle movement that allows a conscious control of the shoulder position.\u00a0 As a result, learning and training may influence movement pattern.\u00a0 This results in large differences among individual people.\u00a0 Anatomic peculiarities, such as chest curvature or dissimilar winging angle (ranging from 30\u00b0-45\u00b0) contribute to this variability.<\/p>\n

Because the scapula has no definite resting orientation, its attitude is defined with regard to a virtual reference position.\u00a0 This is a position in which the scapular plane is parallel to the frontal plane and the spine of the scapula is along the frontal axis.\u00a0 Note that the virtual position is physically not attainable, rather it serves only as a virtual reference position to compare against.<\/p>\n

Generally, scapular movement is described either relative to the sternum or relative to the trunk (a longitudinal axis of the sternum system is inclined to the vertical axis of the trunk by approximately 15\u00b0).\u00a0 A product of two transformation matrices is calculated.\u00a0 The first matrix defines the orientation of the clavicle relative to the sternum and thorax, and the second the orientation of the scapula with regard to the clavicle.\u00a0 In total 6 Euler\u2019s angles (or 18 direction cosines) would be needed for such a calculation.<\/p>\n

Sternoclavicular Joint Kinematics<\/em><\/span><\/p>\n

Kinematically, the sternoclavicular joint functions as a ball-and-socket joint with three DOF.\u00a0 The movements of the clavicle are: elevation\/depression (the lateral end of the clavicle moves up or down); protraction\/retraction (back-to-front and front-to-back movement), and axial (anterior\/posterio) rotation.\u00a0 The fairly large range of clavicular motion in elevation is on average 45\u00b0, in depression it is 15\u00b0, in protraction-retraction it is 30\u00b0, and in axial rotation it is 30\u00b0-45\u00b0.\u00a0 During circumductory motion, the acromial end of the clavicle sweeps an ellipse with the major axis in the inferior-superior direction.\u00a0 The clavicle operates as a bony strut with two articulations at the ends.\u00a0 The clavicular motions are coupled with movements of the scapula (except during pure scapular abduction\/adduction).<\/p>\n

\"sternoclavicular\"<\/a><\/p>\n

Scapulohumeral Rhythm of the Shoulder Complex<\/em><\/span><\/p>\n

For truly efficient movement patterns the body needs the scapula and humerus to work synergistically. \u00a0The term for this is\u00a0scapulohumeral rhythm<\/em>. \u00a0Mike Reinold<\/a>\u00a0says\u00a0normal scapulohumeral rhythm requires a sequence of shoulder and scapular movement simultaneously. \u00a0<\/em>The following video provides a good animation of scapulohumeral rhythm.<\/p>\n